Drug Interactions between codeine / phenylephrine / promethazine and efavirenz

GENERALLY AVOID: Efavirenz can cause significant QT interval prolongation in patients with the CYP450 2B6 *6/*6 genotype due to reduced metabolic clearance of the drug stemming from decreased levels of functional CYP450 2B6 isoenzyme. Theoretically, coadministration with other agents that can prolong the QT interval may result in additive effects and increased risk of ventricular arrhythmias including torsade de pointes and sudden death. In a QT study conducted in 58 healthy subjects enriched for CYP450 2B6 polymorphisms, mean efavirenz peak plasma concentration (Cmax) in subjects with the CYP450 2B6 *6/*6 genotype was 2.25-fold that observed in subjects with the CYP450 2B6 *1/*1 genotype following administration of efavirenz 600 mg daily for 14 days. A positive relationship between efavirenz concentration and QTc prolongation was observed, with mean QTc prolongation and its upper bound 90% confidence interval at 8.7 ms and 11.3 ms in subjects with the CYP450 2B6 *6/*6 genotype. In general, the risk of an individual agent or a combination of agents causing ventricular arrhythmia in association with QT prolongation is largely unpredictable but may be increased by certain underlying risk factors such as congenital long QT syndrome, cardiac disease, and electrolyte disturbances (e.g., hypokalemia, hypomagnesemia). In addition, the extent of drug-induced QT prolongation is dependent on the particular drug(s) involved and dosage(s) of the drug(s).

MANAGEMENT: Alternatives to efavirenz should be considered in patients receiving other drugs that can prolong the QT interval. Patients treated with any medication that can cause QT prolongation should be advised to seek prompt medical attention if they experience symptoms that could indicate the occurrence of torsade de pointes such as dizziness, lightheadedness, fainting, palpitation, irregular heart rhythm, shortness of breath, or syncope.

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MONITOR: Central nervous system- and/or respiratory-depressant effects may be additively or synergistically increased in patients taking multiple drugs that cause these effects, especially in elderly or debilitated patients. Sedation and impairment of attention, judgment, thinking, and psychomotor skills may increase.

MANAGEMENT: During concomitant use of these drugs, patients should be monitored for potentially excessive or prolonged CNS and respiratory depression. Cautious dosage titration may be required, particularly at treatment initiation. Ambulatory patients should be counseled to avoid hazardous activities requiring mental alertness and motor coordination until they know how these agents affect them, and to notify their physician if they experience excessive or prolonged CNS effects that interfere with their normal activities.

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MONITOR: Coadministration of codeine with potent or moderate CYP450 3A4 inducers may result in lower codeine plasma concentrations, higher levels of the inactive metabolite norcodeine, and less metabolism via CYP450 2D6, resulting in lower morphine levels. This interaction may lead to reduced codeine efficacy and potentially initiate the onset of withdrawal symptoms in patients who are physically dependent. In addition, patients may be at an increased risk of CNS and/or respiratory-depressant effects from increased levels of codeine once concomitant therapy with the CYP450 3A4 inducer is ceased. This is particularly worrisome if the CYP450 3A4 inducer also possesses CNS- and/or respiratory-depressant effects. This interaction has also been reported with dihydrocodeine.

MANAGEMENT: The potential loss of efficacy of codeine or dihydrocodeine and onset of opioid withdrawal symptoms should be considered when used in combination with a potent or moderate CYP450 3A4 inducer. Alternative agents with no or minimal CYP450 3A4 induction potential are recommended whenever possible. Some manufacturers of products containing codeine advise against the concomitant use of codeine with CYP450 3A4 inducers. If concomitant use is considered necessary, caution and close clinical and laboratory monitoring are recommended. Dosage adjustments may also be required whenever a CYP450 3A4 inducer is added to or withdrawn from therapy. Following discontinuation of a CYP450 3A4 inducer, patients should be monitored for potentially excessive or prolonged CNS and respiratory depression.

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